I tried to create a working Python code and I considered that you have many SQLite DBs with just one layer (same name as the DB file in lowercase) in your folders `r1` and `d1`.

Your databases are SQLite DB not spatially aware and I loaded the layers with `ogr` and assign a projection, in your case the OSGB 1936 / British National Grid **EPSG: 27700**, to tell what coordinate reference system (CRS) layers have.

Once QGIS know the projections, it can do the <kbd>Service area</kbd> processing and export geometries.

Here is my working code under QGIS 3.16 (Windows, but I took your question OSX paths for the code below) :

<!-- language: python -->
```
import os
import processing
import glob
import string

#Folder where my road networks are stored
Select_folderRoads='//Mac/Home/FINAL/March2021/r1/'
#Folder where the starting points/nodes are stored
Select_folderSDH='//Mac/Home/FINAL/March2021/d1/'
#Folder where the output is to be stored
Save_results='//Mac/Home/FINAL/March2021/path250m/'
# Set working directory to roads folder
os.chdir(Select_folderRoads)

uk_crs = QgsCoordinateReferenceSystem(27700)

# iterate over roads DB network layers
for dbname in glob.glob("*.sqlite"):
    # get layer name from SQLite DB name
    lyr_name = dbname.split(".sqlite")[0].lower()
    # get roads and points layers, set their CRS
    roads_layer = QgsVectorLayer(f"{Select_folderRoads}{dbname}|layername={lyr_name}", "roads_lyr", "ogr")
    roads_layer.setCrs(uk_crs)
    points_layer = QgsVectorLayer(f"{Select_folderSDH}{dbname}|layername={lyr_name}", "points_lyr", "ogr")
    points_layer.setCrs(uk_crs)

    # processing algorithm that output result as GPKG
    result = processing.run(
        "native:serviceareafromlayer",
        {'DEFAULT_DIRECTION': 2,
        'DEFAULT_SPEED': 50.0,
        'DIRECTION_FIELD': '',
        'INCLUDE_BOUNDS': False,
        'INPUT': roads_layer,
        'OUTPUT_LINES': f"{Save_results}{lyr_name}",
        'START_POINTS': points_layer,
        'STRATEGY': 0,
        'TOLERANCE': 0.0,
        'TRAVEL_COST': 250.0,
        'VALUE_BACKWARD': '',
        'VALUE_BOTH': '',
        'VALUE_FORWARD': ''}
    )
```